[jira] [Created] (SPARK-18631) Avoid making data skew worse in ExchangeCoordinator

["Mark Hamstra (JIRA)" <ji...@apache.org>]

Mark Hamstra created SPARK-18631:
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             Summary: Avoid making data skew worse in ExchangeCoordinator
                 Key: SPARK-18631
                 URL: https://issues.apache.org/jira/browse/SPARK-18631
             Project: Spark
          Issue Type: Bug
          Components: SQL
    Affects Versions: 2.0.2, 1.6.3, 2.1.0
            Reporter: Mark Hamstra
            Assignee: Mark Hamstra


The logic to resize partitions in the ExchangeCoordinator is to not start a new partition until the targetPostShuffleInputSize is equalled or exceeded.  This can make data skew problems worse since a number of small partitions can first be combined as long as the combined size remains smaller than the targetPostShuffleInputSize, and then a large, data-skewed partition can be further combined, making it even bigger than it already was.

It's a fairly simple to change the logic to create a new partition if adding a new piece would exceed the targetPostShuffleInputSize instead of only creating a new partition after the targetPostShuffleInputSize has already been exceeded.  This results in a few more partitions being created by the ExchangeCoordinator, but data skew problems are at least not made worse even though they are not made any better.



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